In Operando FTIR Spectroscopy for Lithium-Ion Batteries

Introduction

Lithium-ion batteries have been staying at the center of the advanced battery technologies after the first production by SONY in 1991.  Recent R&D activities for the advanced lithium-ion battery are focusing on vehicle applications such as hybrid electrical vehicles (HEV), plug-in hybrid electrical vehicles (PHEV) and electric vehicles (EV).  Compared with the consumer electronic devices such as mobile phones or laptop computers, the average lifetime of a vehicle is much longer, hence the lithium-ion batteries for vehicle application requires much linger cycle life and calendar life.  Since diagnostic studies of the batteries are crucial for the improvement of the cycle life and the calendar life of the batteries, vigorous analytical studies were carried out to understand the degradation mode of the lithium-ion batteries. 

Electrochemical in situ spectroscopies are powerful analytical techniques for the degradation study of the batteries.  However since the in situ spectroscopy cell always has to have some special design, sometimes the operation mode of the in situcell is different from that of the practical battery system. 

In situ FTIR spectroscopy is one of the popular analytical techniques to characterize the surface film of the electrodes.  Especially the technique is very useful for studying the decomposition process of the electrolyte solutions.  One of the biggest challenges in the in situ FTIR spectroscopy of lithium-ion battery materials is preparation of the thin film electrode to obtain enough reflection of the infrared beam [1].  Furthermore, in the case of the internal reflection geometry, the thin film electrode has to face to the window material; as a consequence high rate charging-discharging process is not applicable for the in situcell.   

In the present work, we designed a new in situ FTIR cell based on a diamond-ATR optical system.  A common battery testing conditions were applicable during the in situ FTIR measurements and various common battery materials such as LiCoO₂ and graphite can be used as composite electrode form, containing electron conductive materials and binder, without any special treatment.  Therefore here we introduce the new measurement technique as “in operandoFTIR spectroscopy” for lithium ion batteries. 

Results and Discussions

All the spectra were taken at single beam mode and Subtractively normalized interfacial FTIR spectra were calculated.  Fig. 1 shows in operando FTIR spectra for LiCoO₂ composite electrode operated at C/3 constant current charging-discharging process (top) associated with the charging-discharging profile (bottom).  Significant spectrum changes were observed at the beginning of the charging process and at the end of the discharging process.  We suspect this spectrum change is initiated by the semiconductor-metal transition of LiCoO₂.  The spectra were relatively stable in the middle of the charging or discharging process.  Another spectrum changes were observed at the end of charging process.  It suggests the electrolyte decomposition occurs at high electrode potential above 4.3 V vs. Li. 

More in operandoFTIR measurement results for various electrode materials will be presented in the meeting.  

Reference

[1] M. Matsui, K. Dokko and K. Kanamura, Journal of The Electrochemical Society, 157 (2010) A121-A129